1. Field of the Invention
The present invention relates to a magnetic core which is formed of substantially flat, elongated, rectangular particles which are termed "microlaminations". These microlaminations are formed from plain carbon steel by cutting the same into a discretely-shaped particle (an elongated parallelopiped of generally rectangular cross-section) following which the microlaminations are decarburized, magnetically insulated and thereafter placed in a mold and pressed to the desired density, said pressing being effective without the use of a binder for producing the finished unitary magnetic core.
2. Description of the Prior Art
Magnetic cores for medium and high frequency uses are often produced from powdered metal magnetic materials or flakes, the latter being usually made by rolling powder particles to a plate-like structure. The purpose of using powders or flakes is to provide increased temperature stability and decreased electrical losses when subjected to high frequency eddy currents because air gaps and insulation are present between the individual particles of the powders or the flakes. Air gaps are desirable to decrease losses but undesirable in another respect since they lower the effective permeability because there is a lower volume of magnetic material in each unit core volume.
Some of the disadvantages of the lower permeability present in powdered iron cores is overcome by fabricating the cores from flakes which yield a higher density or packing factor. Siginificantly, the flakes also improved the Q factor at lower frequency. The Q factor is defined as the ratio of the reactance of the core to the effective series resistance of the core. Generally speaking, the flakes are insulated and flat, the length and width dimensions being much larger than their thickness.
In U.S. Pat. No. 2,689,398 to G. C. Gaut et al, there is disclosed a method of making magnetizable compacts. As there set forth, the magnetic material in fine particulate or powder form is fed to a pair of flaking rollers where the material is flattened to a flake thickness usually in the range between about 0.015 and about 0.025 millimeter in thickness and transverse dimensions of from about 0.02 to about 0.5 millimeter. Following flattening, the flaked material is mixed with silica powder to separate the flakes, and the mixture is heated to a temperature of about 900.degree. C in order to remove the strains induced during the cold working of the particulate material through the flaking rolls. Advantageously, hydrogen or cracked ammonia is utiliized to prevent oxidation of the materials during said annealing processing. After annealing, the flakes, in the soft condition, are transferred to a separator for removing the silica and thereafter, preferably the flakes are then treated to provide an oxidized surface thereon by heating in air to a temperature within the range between about 200.degree. and 250.degree. C. The patentees found, however, that the presence of an oxide surface on the flakes is not essential. Thereafter, the flakes were charged into a die of predetermined configuration and the flakes were preferably so disposed in the die to assume positions in which they lie parallel to the magnetic lines of force to be set up in the core during subsequent use. Punches were applied to the die so as to exert a pressure of between 15 to 30 tons per square inch to the faces of the flakes, thereby compressing them into the final core configuration.
Other workers in the field as typified by U.S. Pat. No. 3,255,052 to Opitz, follow essentially the same route, namely starting with a metallic powder and thereafter rolling the same to form a flake which is ultimately utilized within a core configuration. Notably different with Opitz is the fact that he requires each of the individual flakes to be magnetically insulated by a plurality of coatings which coatings permit the finally configured core to be annealed at a high temperature without destroying the magnetic flake insulation. Thus, after the core was finally formed, the same was annealed at a temperature between 800.degree. to about 950.degree. C to relieve the work strains and produce the desired magnetic characteristic. Following heat treatment the cores were quenched at a rate of between about 15.degree. and about 75.degree. C per minute. While Opitz is directed to a method particularly adapted to that material known commercially as molybdenum permalloy, it is stated that the process is also effective when used with other materials.
Other patentees, namely Adams et al. in U.S. Pat. No. 2,937,964, also use molybdenum permalloy and teach a method for melting the composition as well as formulating the powder, flaking, annealing, insulating, aligning and compacting the same. This is followed by another final annealing of the finished core at a temperature within the range between about 600.degree. and about 700.degree. C and the annealed core is thereafter quenched in air to room temperature.
Thus, from the foregoing practices, it becomes clear that the preferred method of the prior art was to employ metallic particles either spheroidal or irregular in shape and flattening the same to form the individual flake-like laminates going into the core. Opitz teaches that the permeability of the magnetic core will vary with the flake diameter and the core losses varies with the flake thickness, both increasing with increases in their respective dimensions.
Roseby, U.S. Pat. No. 1,850,181 suggests that cores may be prepared by drawing fine wire preferably of a nickel-iron alloy, of a diameter of 4 mills, cutting the wire into small lengths, which wire is annealed, coated with an iron-phosphate, then coated with an insulating varnish and finally pressed to 10 to 15 tons per square inch into a core. In order to fill the relatively large spaces between the wire lengths, Roseby states that up to 20 % of powdered iron be added to produce a more compact core. Fine wire is extremely expensive to produce, and as Roseby indicated, the sections of wire have such a poor packing or space factor that even if the round wire sections are perfectly arranged, at least up to 20 % of powdered iron should be added to fill some of such spaces in producing a more efficient core.
In contrast thereto, the applicants' present invention employs a heretofore commercially available, flat worked sheet material in a new and different manner and by forming discretely-shaped microlaminates, the several improvements and benefits of particulate cores can be obtained using microlaminations in the manner which will appear more fully hereinafter.